The present invention relates generally to a power supply unit for automotive vehicles. More specifically, the invention relates to a power supply for feeding electric current to various electrical loads of an automotive vehicle.
In recent years, it has been proposed to use an electric double layer capacitor, the capacity of which is remarkably increased using an electric double layer structure of electrons produced on the interface between electrodes and an electrolyte, as a cell. This electric double layer capacitor generates a voltage of, e.g., about 2.5 V, as a single cell. Therefore, in a case where this electric double layer capacitor is used for a power supply unit for an automotive vehicle, a plurality of single cells 100 are connected in series to be used as a capacitor pack as shown in FIG. 1.
That is, each of the single cells 100 is formed as an electric double layer capacitor which comprises a pair of current collecting bodies, each having an active carbon electrode, an electrolyte charged between the current collecting bodies, and a separator for separating the active carbon electrodes of the respective current collecting bodies from each other (all the elements are not shown). These single cells 100 are connected in series to form a capacitor pack 102. The capacitor pack 102 is connected to an on-vehicle generator 104 mounted on the vehicle. The on-vehicle generator 104 generates a voltage according to engine rotation, and the generated voltage is converted into a DC voltage by means of a rectifier to be output. Specifically, an alternator corresponds to the on-vehicle generator 104.
During the engine is operated, each of the single cells 100 is charged by the DC voltage from the on-vehicle generator 104, and the stored energy is discharged and used to drive a starter motor when the engine is restarted, and so forth.
The electromotive force of each of the single cells 100 is determined by the active voltage of the electrolyte. When a bias voltage, i.e., a charging voltage from the on-vehicle generator 104, exceeds the active voltage, the life time of the single cell 100 of electric double layer capacitor is rapidly decreased. In view of a margin of the safety, if the rated voltage of each of the single cells 100 is set to be lower than the active voltage of the electrolyte and if the single cells 100 are used at a voltage equal to or less than the rated voltage, it is possible to ensure a long life time which is one of the characteristics of the electric double layer capacitor.
However, the dispersions in electrostatic capacity and internal resistance of the single cells 100 may occur. Therefore, if the single cells 100 are charged from the on-vehicle generator 104 while being connected in series, the difference between the bias voltages may occur due to the dispersions in electrostatic capacity and internal resistance. That is, in a case where the single cells 100 are connected in series to be charged, if the electrostatic capacities and the values of internal resistance of the respective single cells are the same, the terminal-to-terminal voltages (the bias voltages) of the respective single cells are also the same. However, if there are the dispersions in electrostatic capacity and internal resistance, the imbalance in bias voltage occurs between the respective single cells. In addition, this difference in bias voltage may be integrated and increased when charge and discharge are repeatedly carried out.
Therefore, if the rated voltage is set in view of a margin of safety, a higher bias voltage than the rated voltage may be impressed on one of the single cells 100 due to the dispersion in characteristics of the respective single cells 100, which may cause a decrease in the life time.
In order to overcome such a problem, a "balance circuit system" has been adopted. As shown in FIG. 2, this system is designed to balance the bias voltages of the respective single cells 100 by connecting balance resistors 106 having the same resistance to the respective single cells 100 via resistors 108 in parallel for every single cell 100 and by connecting the respective balance resistors 106 in series.
In addition, Japanese Utility Model Laid-Open No. 5-23527 discloses another conventional system wherein a balance resistor is electrically separated from a single cell when no charge is carried out, and the single cell and the balance resistor are connected in parallel when the terminal-to-terminal voltage of the single cell is higher than a predetermined value.
In the aforementioned conventional balance circuit system wherein the balance resistors 106 are connected to the respective single cells 100 in parallel, it is possible to equalize the bias voltages applied to the respective single cells 100, since the divided voltages are the same if the values of resistance of the respective balance resistors 106 are the same.
However, in the case of the balance circuit system, since the balance resistors 106 are always connected to the respective single cells 100 so that the whole electric circuit forms a closed loop, the electric energies stored in the respective single cells 100 are gradually lost as the discharge is carried out. Therefore, this system can not function as a "battery" since the discharge of each of the single cells 100 are started after the charge is stopped, so that this system can not be practically used as a power supply unit for driving, e.g., a starter motor, which is used when the engine is restarted, of an automotive vehicle.
In order to overcome this problem, in the aforementioned system disclosed in Japanese Utility Model Laid-Open No. 5-23527, the balance resistors are selectively connected to the single cells in parallel and separated therefrom by means of transistors and so forth. According to this system, it is possible to prevent the discharge to store the electric energy for a long time, since the balance resistors are separated from the single cells when no charge is carried out. However, according to this system, there are other problems in that the circuit structure is not only complicated due to the additional switch circuit to increase the manufacturing costs, but the reliability is also decreased due to the increase of the number of parts.
The aforementioned problems on the balance circuit system also appear in a lead battery pack used for a power supply unit of an electric vehicle. This lead battery pack is made by connecting a plurality of single cells like the capacitor pack shown in FIGS. 1 and 2.